Grave situation's beginning: euro is destabilised (7)

I believe the answer is:

serious

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'grave' is the definition.
(I know that grave can be written as serious)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for serious that I've seen before include "Not light or frivolous" , "Important -- grave" , "Important - solemn" , "Earnest; vital" , "Solemn, thoughtful" .)

I've seen this clue in The Sun.

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